This invention relates, in general, to semiconductor processing, and more particularly, to a method of fabricating a buried insulating layer.
Silicon On Insulator (SOI) structures are comprised of a substrate, an insulating layer on the substrate, and a superficial semiconductor layer on the insulating layer. SOI structures are used to electrically isolate an integrated circuit fabricated on a thin silicon layer from the substrate. The buried insulating layer electrically isolates the substrate from the superficial semiconductor layer. SOI structures can be formed by a Separation by IMplantation of OXygen (SIMOX) method. A typical SIMOX method comprises implanting oxygen into a silicon substrate or bulk silicon wafer. The oxygen reacts with the silicon to form a silicon dioxide layer during the implantation of oxygen and during a subsequent thermal anneal. The silicon dioxide layer which is formed in the substrate is called a buried silicon dioxide (oxide) layer. The silicon left at the surface is called a superficial silicon layer.
It is desirable to have a thick buried oxide layer in order to lower the capacitance and increase the breakdown voltage between the superficial silicon layer and the substrate. The capacitance and breakdown voltage of the buried oxide layer are important characteristics to control especially when bipolar or power devices are formed on the SOI structure. A buried oxide layer formed by using the conventional SIMOX method typically is limited to a thickness of approximately 0.3 microns. The capacitance of a buried oxide layer of this thickness may be too high for certain applications, and thus it would be desirable to increase the thickness in order to lower the capacitance. In addition, the breakdown voltage of the buried oxide layer of this thickness is low for high voltage power devices.
A way of increasing the thickness of the buried oxide layer has been achieved by implanting a very high dose of oxygen into the substrate. This method is unacceptable, however, because the resulting dislocation density in the superficial silicon layer is proportional to the implant dose. In addition, the thickness of the superficial silicon layer can not be controlled separately from that of the buried oxide layer. When a high oxygen dose is used, the thickness of the superficial semiconductor layer decreases and the dislocation density increases.
By now it should be appreciated that it would be advantageous to provide an alternative method of forming a thick buried insulating layer in order to improve the breakdown voltage and reduce the capacitance of the buried insulating layer.
Accordingly, it is an object of the present invention to provide an improved method of forming a thick buried insulating layer.
Another object of the present invention is to provide a method of increasing the breakdown voltage of a buried insulating layer.
A further object of the present invention is to provide a method of reducing the capacitance of a buried insulating layer.
Yet another object of the present invention is to provide a method of forming a thick buried insulating layer with a substantially lower density of dislocations induced by the implantation of oxygen.
Yet a further object of the present invention is to provide a method of controlling the thickness of a buried insulating layer separately from that of the superficial silicon layer.
Still another object of the present invention is to provide a method of forming multiple buried insulating layers.